The present invention relates to an automatic welding machine that operates with rod electrodes. The electrodes, which are held in a welding head and can be shifted about with respect to the workpiece, are moved longitudinally to the workpiece by the welding head in accordance with the arc voltage so as to sustain a certain arc length. After use, the electrodes are replaced automatically with new electrodes. An automatic welding machine of the type described above is generally disclosed in the German Patent Publication No. 36 19 761. In this prior art machine the voltage-dependant longitudinal movement of the electrodes is said to be performed under analog control by means of two electric motors--a DC motor and an AC motor--acting through a differential drive on a threaded spindle. The automated performance of arc welding work with welding rods is especially desirable where high-grade welds of unvarying quality are to be made in great number on one and the same workpiece. An example of such work is the permanently leak-proof welding of tube ends into header plates. With the present state of the art as many as 25,000 tubes are welded in a single unit.
The above-mentioned type of arc-welding machine leaves certain problems unsolved, however, with respect to practical applications. What has proven especially difficult in automated operations is the reliable striking of the arc at a predetermined location. For one thing, especially in the case of overlapping beads, contact can be made uncertain by scale or by slag such as develops from the electrode coating. Another problem is the danger of an electrode sticking, in the manner familiar to every manual arc welder.